"Persistent" weak layers take a notoriously long time to stabilize after a storm.  Persistent weak layers include faceted snow, depth hoar (faceted snow at depth) and surface hoar (frost).  (from Staying Alive in Avalanche Terrain, by Bruce Tremper - Mountaineers Books)

If we overlay the number of fatalities that occur during each danger rating, we can see why most of them occur in terrain rated as Considerable and Moderate and most occur on persistent weak layers.  

It's not so much of an avalanche problem as a people problem.  People are very impatient.  As the danger rating decreases, people's courage increases. Thus, the maximum interaction between people and avalanches occurs at Considerable danger.  81% of fatalities occur in terrain rated as either Moderate or Considerable.